A combined physiological and comparative approach to the definition, measurement and analysis of sex differences in behavior of various species. Physiological studies will deal with contributions of sex hormones to early development of such differences and to their expression in adult life, as well as the identity and functional characteristics of brain mechanisms mediating sexually dimorphic patterns. The comparative studies will focus on analysis of sexual "attractivity" and "proceptivity", i.e. aspects of feminine sexuality going beyond mere "receptivity" to the male. Effects of hormones on organization of the brain during early development will be examined by extension of ongoing studies on prenatally masculinized female dogs, plus addition of a new group of prenatally demasculinized males born of mothers treated with antiandrogen during pregnancy. Neurophysiological mediation of sexually dimorphic behavior in dogs will be studied in males subjected to anterior hypothalamic-preoptic lesions before puberty. Types of behavior to be examined include mating, social dominance, urinary frequency and posture, production and response to pheromones normally involved in canine sociosexual interaction. Attention will be given not only to the manifestation of sexually dimorphic patterns in adulthood, but also to the development of sex differences from infancy to puberty.